Apparatus for control of bias potential in an electrophotographic copier

ABSTRACT

An apparatus and method are disclosed for adjusting the potential difference between the conducting layer and ground at any preselected position on a photoconductive element that is continuous from exposure through development stations.

United States Patent Morse 1541 APPARATUS FOR CONTROL OF BIAS POTENTIALIN AN ELECTROPHOTOGRAPHIC COPIER [72] Inventor: Theodore H. Morse,Rochester, N.Y.

[73] Assignee: Eastman Kodak Company, Rochester,

[22] Filed: Mar. 9, 1970 [21] Appl. No.: 17,394

521 u.s.c| ..355/10,96/1A,l17/37LE, 355/16 51 lnt.Cl. ..G03g 13/10..355/3, l0, l6, l7;96/l R; 117/37 LE; 250/49.52 C; 96/1 A, l C

[58] Field of Search 51 Mar. 21, 1972 [56] References Cited UNITEDSTATES PATENTS 3,456,109 7/1969 Gawron ..L ..250/49.5

3,368,894 2/1968 Matkanetal ..l17/37 LE Primary Examiner-Joseph F.Peters, Jr.

Assistant ExaminerRichard L. Moses Attorney-W. H. J. Kline, Paul R.Holmes and J. William Berkstresser [5 7] ABSTRACT An apparatus andmethod are disclosed for adjusting the potential difference between theconducting layer and ground at any preselected position on aphotoconductive element that is continuous from exposure throughdevelopment stations.

2 Claims, 3 Drawing Figures Patented March 21, 1972 3,650,622

- INSULATING LAYER "CO/VDUC Tl/VG LA YER ""INSULA 7'//VG SUPPORT IaR0u/v0 PLANE FIRST CHARGER SECOND CHARGER 33 v 36% 1 T f t f l l-//V.SULA7'//V6 LArER CONDUCT/N6 LAYER 'l/VSULA TING SUPPORT GROU/V0PLANE ////F-.35

THEODORE H. IMO/PS5 INVENTOR.

yurfi/m A TTOR/VEY APPARATUS FOR CONTROL OF BIAS POTENTIAL IN ANELECTROPIIOTOGRAPIIIC COPIER This'invention relates to the predeterminedcontrol and adjustment of the potential of a conducting layer in aphotoconductor element.

It is desirable in electrophotographic imaging processes that thecharging and developing operation be carried out under optimumelectrical conditions. That" is, the state of charge achieved on thesurface of a photoconductor element as well as the performance of theelement during development and transfer operations dependsv upon thepotential difference between the conducting layer contained, within thephotoconductor element and ground. In most photoconductive elements theconducting layer is simply grounded to the apparatus or other convenientground. In xerographic operations such as disclosed in L. E. Walkup U.S.Pat. No. 2,934,649 where a selenium photoconductive insulating layer isused on a conducting metal drum, the drums potential'is not normallyadjustable between the operations of charging, exposing, development andtransfer.

Normally, adjustment of the bias of the conducting layer of aphotoconductor containing continuous web has been limited to alteringthe value of the ground potential. Since the conducting layercontinuously underlies the photoconductive layer, adjusting the bias ofthe conducting layer to optimum values at more than one pointsimultaneously, has not'hitherto been possible.

Various disclosures in the xerographic imaging art are directed towardinductive or noncontract adjustment of potential differences. Forexample, R. W. Gundlach, US. Pat. No. 2,965,481 issued Dec. 20, I960discloses a typical dual corona charging setup for providing therequisite potential difference betweenone side of a photoconductiveelement and the other. Likewise. Gundlach US. Pat. No. 2,885,556issuedMay 5, I969; F. A. Steinhilper US. Pat. No. 2,955,938 issued Oct. ll,I960 and S. Naroff U.S. Pat. No. 3,4 1 L846 issued Nov. l9, I968disclose similar apparatus. Likewise, E. C.

Giaimo'US. Pat. Nos. 2,815,449 and 2,922,883 describe charging processesutilizing two chargers mounted on oppositc sides of the photoconductiveelement. These latter two patents describe the use of photoconductiveelements which do not contain a conducting layer.

It is an object of the present invention to provide a method andapparatus for biasing the conducting layer of a moving photoconductivebelt or web in a manner which enables selection of predeterminedpotential differences between the conducting layer and ground, which areoptimum for the various operations which must take place in theproduction of electrophotographic images.

The foregoing object and hereinafter described advantages are achievedaccording to the present invention by providing dual charging meanswhich can be located on the photoconductor-containing side of theelement in spaced relation from each other. These charging means arethen operated at predetermined potentials at opposite polarities.

A preferred form of the invention is shown in the accompanying drawingwherein;

FIG. 1 shows the schematic of an electrophotographic duplicationapparatus embodying the present invention.

FIG. 2 is a cross section of a normal photoconductive element showingcharge patterns which occur during electrophotographic use of theelement when the conducting layer is not grounded.

- FIG. 3 is the same as FIG. 2 with charge patterns produced by themethod and apparatus of the present invention.

A typical xerographic processing system with which the present inventionmay be utilized is shown in FIG. 1 in which the various systemcomponents are schematically illustrated. A general understanding ofoperation of the present invention can be derived from examination ofsuch a system, the teachings of course being applicable to otherxerographic systems employing these same principles. As in allxerographic systems based on the concept disclosed in the US. Pat. toCarlson, No. 2,297,681, issued Oct. 2, 1942, a pattern of actinicradiation representing the copy to be reproduced is projected onto thecharged surface of a photoconductive element toform an electrostaticcharge pattern thereon. Typically, the electrostatic charge pattern isdeveloped with an oppositely charged developing material to form a tonerimage, corresponding to the electrostatic charge pattern, on the surfaceof the photoconductive element. The developed image is then typicallytransferred onto a second surface where it may be fused by a variety offusing devices, such as heaters and the like, whereby the image iscaused to adhere to the second surface permanently. Alternatively, theelectrostatic charge pattern can be developed by liquid developmenttechniques with or without the necessity of afusing station. Likewisethe toner image can be developed 'to a permanent'or semipermanent formon the surface of the photoconductor element.

Referring now'specifically to the drawings, FIG. 1 shows a schematicillustration of an electrophotographic duplicating device utilizing aphotoconductor-containing web moved by a roller transport mechanism. Thephotoconductor containing web 12 is introduced to the mechanism from asupply roll I4 and rewound onto pickup roll '16.Photoconductor-containing web 12 is transported around the mechanism bymeans of transport rollers 18 and developing station rollers 20. A highvoltage corona charger 30 is stationed before exposing station 22 and asecond high voltage charger 30 is located after the developing station24. Exposing station 22 consists typically of a supply reel 26containing film to'be duplicated and a film takeup reel 28 with alight'source 25 and aperture control 27 for control of the exposure ofthe photoconductor-containing web.

The development station 24 typically comprises a vessel containing smallparticles of marking material suspended in an insulating liquid andpreferably utilizes a development electrode 23. In operation, the coronachargers when activated, are set to different polarities and, in theconfiguration shown, establish the proper potential difference betweenthe conducting layer and ground to provide for optimum charging beforeexposing station 22 and optimum development at station 24. Adjustment'ofthe voltages employed as well as the location and general configurationof the particular mechanism used will provide a predictable level ofpotential difference. Such adjustment will not require extensiveexperimentation and therefore should be within the skill of the operatorto perform.

FIG. 2 shows a typical charge pattern distribution in an element havinga photoconductive layer and a conducting layer on a support where asingle charger 33 is used, and no connection made between the conductinglayer and the ground plane 35. Typically a negative charge is applied tothe surface of the photoconductor-containing insulating layer whichinduces positive charges to move in the conducting layer to a positionnear the negative surface charge. The resulting negative charge in otherareas of the conducting layer induce positive charges in the groundplane 35, resulting in an undesirable potential difference between theconducting layer and ground 35.

FIG. 3 shows the charge patterns produced according to the presentinvention wherein the first charger 33 and the second charger 36 areoperated at different polarities. In this manner the conducting layerwill achieve electrical neutrality. However, it can be seen that thelocation of the second charger, being at a specific point. on thephotoconductorcontaining web, will produce a gradation from electricalneutrality to a positive or negative value depending on the positionsensed between the two chargers. This is contrary to the phenomenonoccurring in the photoconductor-containing web shown in FIG. 2 where thecharges will be induced from the ground plane nearest the conductinglayer. The following are examples of the operation of the method andapparatus of the present invention.

EXAMPLE 1 A 35 mm. photoconductor-containing web comprising a polyestersupport having coated thereon a cuprous iodide conducting layer, abarrier layer, and an'unsensitized photoconductor-containing insulatinglayer, was suspended between two insulating posts to provide electricalisolation from ground. Two corona chargers were placed facing thephotoconductor-containing surface of the web and a distance of 3 feetfrom each other. An electrometer probe was positioned above thephotoconductor-containing web surface, midway between the coronachargers so that the potential of the conducting layer could bemeasured. With one corona generator energized negatively and the secondcharger inoperative the conducting layer could be made to assume anegative polarity of several hundred volts by adjusting the intensityofthe first charger. The second charger was then energized positivelyand its intensity gradually increased. The conducting layer potentialwas observed to rise steadily form its negative value as the coronaintensity was increased. A second-charger setting was reached at whichthe conducting layer reached zero potential, when the second chargerpotential was increased further the conducting layer potential could berendered positive. A maximum positive conducting layer potential ofapproximately 200 volts was achieved by this method.

EXAMPLE 2 Utilizing the same setup as described in Example 1 the secondor positive corona charger was moved so that it faced the edge of thephotoconductor-containing web. The edge of the cuprous iodide layer wasthen exposed to the output of the charger. With this setup the intensityrequired to produce a conducting layer potential of zero was less thanrequired when the charge was facing the photoconductor-containingsurface. It was also possible to achieve a positive conducting layerpotential of 400 volts.

Example 3 Using the isolated photoconductor-containing web, electrometerprobe, and chargers as described in the previous examples, a chartrecorder was connected to record the electrometer readings as a functionto time. The two chargers were energized and adjusted to bring theconducting layer potential to zero. The chargers were then allowed tooperate for a 5-minute period without adjustment. The chart recordindicated a maximum deviation of volts from zero during EXAMPLE 4 Usinga roller transport mechanism as shown in FIG. I, the second charger wasadded over the photoconductive surface at a point following thedevelopment station 24. With the first charger 30 adjusted to apply a1000-volt potential to the photoconductor surface, the second chargerwas set to adjust the conducting layer potential to zero at thedevelopment station. In operation, the photoconductor-containing belt orweb was moved at 1 foot per second while charging, exposure anddevelopment were carried out without the second charger energized. Thedeveloped elements had a high level of background toning indicating thatthe conducting layer was rising to a potential of the same polarity asthe image charge. Then without stopping the belt, the second charger wasenergized, bringing the potential of the conducting layer to zero. Thedeveloped elements which resulted from this change had greatly reducedbackground densities compared to the single charger run.

In the practice of the method of the present invention a variety ofmodifications are possible. Since control of the potential isparticularly difficult in the case of an overcoated conducting layer itis important to have control of the potential at both charging anddevelopment stations. The present invention permits adjustment of thebias for the development station in addition to facilitating thephotoconductive surface charging whether the conducting layer is or isnot exposed. Both ofthese operations can then be performed withoutphysical contact with the conducting layer. For example, it is possibleto position one charger next to the edge of thephotoconductor-containing web shielding the surface of thephotoconductor, if necessary. In this manner the second charger can belocated from a position relatively near to the first charger to a pointanywhere along the front or rear of the belt or web for example to theposition shown after the development station.

Without intending to be bound thereby, the mechanism of the processappears to involve the exchange of charge carriers in the conductinglayer between the two charger sites as hereinbefore described. Theassociated currents which will be produced in a moving web will cause apotential gradient to exist in the conducting layer in the regionbetween the two chargers. The magnitude of the potential gradient isdirectly proportional to the conducting layer resistivity and theintensities of the corona chargers. If the currents induced by these twosources are balanced, the point on the conducting layer which ismidwaybetween the two chargers will be at zero potential. An increase ofreduction of either charger output will change the potentialdistribution along the conducting layer and move the point of zeropotential. It can therefore be seen that a change in either theintensity of the chargers or their location can alter, in apredetermined manner, either the point of zero potential, or of aselected potential, along the moving web. This adjustment may beaccomplished without undue experimentation on the part of the operator.

In the practice of the method of the present invention the mostsuccessful and preferred photoconductor-containing webs or belts orsupports are organic photoconductor-containing elements. Thisdescription covers a very broad class of materials which arecharacterized by their ability to act as insulators until exposed tolight. The same prerequisite is necessary for organometallicphotoconductive materials which are normally dispersed in a resinousmaterial. Useful photoconductors of both types are described in Goldmanand Johnson U.S. Patent application Ser. No. 650,664 filed July 3, 1967,U.S. Pat. No. 3,472,839 and Johnson application Ser. No. 755,711 filedAug. 27, 1968. Especially useful class organic photoconductors arereferred to as organic amine photoconductors which are usefulphotoconductor-containing elements such as used herein. Such compoundinclude aryl amine compounds such as those described in Fox U.S. Pat.No. 3,240,597 issued Mar. 15, 1966 and the like as well as U.S. Pat. No.3,180,730 issued Apr. 27, 1965 and Fox U.S. Pat. No. 3,265,496 issuedAug. 9, 1966, as well as U.S. No. 3,274,000 and French Pat. No.1,383,461 likewise U.S. application Ser. No. 627,857, now U.S. Pat. No.3,542,544 filed Apr. 3, 1967 discloses useful photoconductors. Suchphotoconductors may, if desired, be used with or without sensitizersdepending on the choice of the designer. Typical sensitizing addenda forimproving speed and/or spectral response as well as other desiredelectrophotographic characteristics are disclosed in U.S. Pat. Nos.3,250,615, 3,141,770 and 2,987,395.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

lclaim:

1. In an electrophotographic copier including means for advancing alonga predetermined path an electrophotographic developing means forapplying an electrical potential to the conductive backing of therecording element to establish an optimum potential in the backing atany preselected position along said path.

2. The invention according to claim 1 wherein said applying meanscomprises a second corona discharge source for directing ions of apolarity opposite said first potential at the recording element, andmeans for varying the discharge intensity of said second coronadischarge source.

PC1-1050 UNITED STATES PATENT OFFICE 569 CERTIFICATE OF CORRECTIONPatent No. 3,65 ,622 Dated Wamch 21 1972 Inventor) Theodore H. Morse IIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

r- In Column 1, line 60, Change duplication", to

"duplicating".

In Column 3, line 19, change "form" to --irom--.

In Column L, line 33, change "of" to --or- In Column 5, line 6, changedissipation to --dissipating--;

' In Column 5, line 9, after "electroscopio", insert --particles to thephotoconductive layer to render the charge.

Signed and sealed this 8th day of August 1972.

(SEAL) Attest:

EDWARD M.FI.,ETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof. Patents V 3 3 I UNITED STATES PATENT OFFICE CERTFICATE OF CORRECTIONPatent No. $65 2 Dated Q C 1972 Inventofls) Theodore I'Se It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[- In Column 1, line 60, change duplication" to --duplicating--..

In Column 3, line 19, change "form" to --from-.

In Column L, line 33, change of" to -or- In Column 5, line 6, change"dissipation" to --dissipating--;

v In Column 5, line 9, after "electroscopic", insert -particles to thephotoconductive layer to render the charge".

Signed and sealed this 8th day of August 1972.

(SEAL) Attest:

EDWARD MQFLETCHER,JR. ROBERT GOT'I'SCHALK Attesting Officer Commissionerof Patents

1. In an electrophotographic copier including means for advancing alonga predetermined path an electrophotographic recording element having alayer of photoconductive material and an electrically conductive backingwhich is electrically isolated from ground, a first corona dischargesource positioned adjacent such path for impinging ions of a firstpotential upon the photoconductive layer to uniformly charge such layer,means for selectively dissipation the uniform charge on thephotoconductive layer to form a developable charge pattern thereon, anddeveloping means positioned adjacent such path for applyingelectroscopic pattern visible, the improvement comprising: meanspositioned adjacent said path downstream from the developing means forapplying an electrical potential to the conductive backing of therecording element to establish an optimum potential in the backing atany preselected position along said path.
 2. The invention according toclaim 1 wherein said applying means comprises a second corona dischargesource for directing ions of a polarity opposite said first potential atthe recording element, and means for varying the discharge intensity ofsaid second corona discharge source.